centos minimal install +xfce +setup tools

nmcli d  

The ONBOOT edit may be performed (as root) and assuming the relevant device is eth0:

# cd /etc/sysconfig/network-scripts/
# sed -i -e 's@^ONBOOT="no@ONBOOT="yes@' ifcfg-eth0
As to "breaking expectations": The foregoing example uses a 'traditionally' named network device of: eth0 Other device names are also possible, including for example: em1 or p3p1 and such. Like it or not, this change in approach in interface naming is the future path for Linux. It was previewed in in the upstream's "testing distribution". See also the materials at: Dell's writeup and a blog post from an insider there.

3. But, I just want it to work and to hand-edit the configuration files

Many installations do not require the complexity of the NetworkManager tool, and use hand-edited configuration files instead. Here is a sample non NetworkManager DHCP interface configuration:

[root@example ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
BOOTPROTO=dhcp
NM_CONTROLLED="no"
PERSISTENT_DHCLIENT=1
ONBOOT="yes"
TYPE=Ethernet
DEFROUTE=yes
PEERDNS=yes
PEERROUTES=yes
IPV4_FAILURE_FATAL=yes
IPV6INIT=yes
IPV6_AUTOCONF=yes
IPV6_DEFROUTE=yes
IPV6_PEERDNS=yes
IPV6_PEERROUTES=yes
IPV6_FAILURE_FATAL=no
NAME="eth0"
[root@none ~]#
and a sample 'static assignment' configuration file:

[username@hostname]$ cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
HWADDR="00:21:70:10:7E:CD"
NM_CONTROLLED="no"
ONBOOT="yes"
BOOTPROTO=static
# BOOTPROTO=dhcp
IPADDR=10.16.1.106
NETMASK=255.255.255.0
#
#   the GATEWAY is sometimes in: /etc/sysconfig/network
GATEWAY=10.16.1.1
and then common items such as hostname and DNS servers may optionally be placed in:

[username@hostname]$ cat /etc/sysconfig/network
HOSTNAME=acme.example.com
DNS1=10.16.1.112
DNS2=8.8.8.8
## DNS2=76.242.0.28
SEARCH=example.com
The information there is 'optional' because a DHCP server can hand out these values. The initscripts are able to figure out hostname and so forth when a well-populated DNS environment exists, from PTR records and such, but some users need to manage such details manually. For more information, the full initscripts documentation files may be listed thus:

rpm -qd initscripts
even in a environment lacking the man manual reading package and its dependencies.
ping google.com
yum instal epel-release

yum --eneblerepo=epel -y groups install "Xfce"

yum check-update

   10  yum update

yum groupinstall X11

yum install setuptool -y

yum install system-config-network* -y

yum install system-config-firewall* -y

yum install system-config-securitylevel-tui -y

yum install system-config-keyboard -y

yum install ntsysv -y